1. Field of the Invention
This invention relates to a method for manufacturing a semiconductor device.
2. Description of the Related Art
Silicon oxide film (SiO2 film) is a gate insulating film which is most universally employed in a semiconductor circuit. Due to the recent trend to further increase the density of semiconductor circuit, the thickness of SiO2 film is required to be decreased. Additionally, since the intensity of electric current leaking the SiO2 insulating film tends to increase, it is getting increasingly difficult to design the transistor or capacitor. The relative dielectric constant of the insulating film may be increased by the addition of an element such as Hf, N or Al to the SiO2 film. Therefore, it is now tried to utilize these elements so as to weaken the electric field strength to be imposed on the insulating film to minimize the leak current even if the SiO2 equivalent film thickness is thin.
Under some circumstances, a p-type impurity in the (poly-Si) gate electrode may diffuse into the gate insulating film. As a result, the gate voltage Vfb (flat band voltage) shifts from the inherent value.
For example, doping poly-Si gate electrode with a p-type dopant such as boron is used in a process which is considered mainly used currently. Through this doping, it is possible to create a transistor having a predetermined work function. It is also proposed to dope the poly-Si gate electrode with indium addition to boron. (JP-A 2004-214673 (KOKAI))
However, in that proposal, the state of dopant atom such as indium is not regulated. Due to the presence of indium, the p-type dopant may be prevented from passing through a silicon layer or an SiGe layer. Under the heat treatment conditions to be employed in the ordinary LSI process for activating the p-type dopant, it would be impossible to electrically activate the indium existing in the silicon layer. Therefore, because of the state of dopant atom such as indium, it is impossible to prevent the generation of depletion layer having a thickness nearly corresponding to the layer containing indium. Thus, the thickness of effective gate insulating film would be increased by a thickness corresponding to the layer containing indium.
Further, if a substance having a high dielectricity is employed as a gate insulating film, the thickness of the depletion layer locating on the gate insulating film side of gate electrode would be increased by about 0.3 to 0.5 nm as compared with the SiO2 film. As a result, the merit of the high dielectricity would not be wasted.